A difference of up to 0.02 in the echelle angle (ECHANGL) or cross disperser angle (XDANGL) between the object, star, flat field, and arc lamp exposures will be accepted by makee. However, a warning will be given for angles larger 0.0021. In general, the angles should not differ by more than 0.0021 to avoid reduction problems. In particular, you can get very bad problems if the XDANGL value differs by much more than 0.002 between your object and flat field, especially if you are using a small slit length which overlaps in the bluest orders.

Note: With 1x1 binning, you will see a change of about 1 column for every 0.001 units in ECHANGL. and about a change of about 1 row for every 0.001 units in XDANGL

It is important to check the XDANGL agreement between all of your object and flat field exposures during your observing run. If you see any large differences, you should take additional flat fields aligned with your object exposures.

If you find out after your observing run that your flat field XDANGL does not agree, you can get additional flat fields at a later date weeks (or months) after your observing run (although this is not optimal since changes in the flat field pattern are possible over long periods of time.)

If the encoder is wrong, i.e. the accurate angles match the values in the header in are wrong, you may need to change the FITS header in your RAW image files. You can edit any FITS header using hdedit .